1. Field of the Invention
The present invention relates to an eye fundus camera and especially to a "non-mydriatic type" eye fundus camera for examining and taking a picture of the fundus of an eye to be examined without application of any mydriatic agent to the eye.
2. Description of the Prior Art
In the "non-mydriatic" type eye fundus camera in which infrared light is used for positioning for fundus observation and visible light is used for taking a picture of the fundus there is provided means for separating the light reflected from the fundus into two beam fluxes one of which is directed to the photographing film and the other toward TV observation system. As such separation means, a total reflection swing-up mirror has conventionally been used. However, to change over the photo-taking light beam of relatively large diameter requires a large swing-up mirror, so that the mechanism becomes complicated and bulky.
It has been proposed to use a dichroic mirror disposed obliquely to reflect infrared light but transmit visible light (U.S. Pat. No. 4,283,124). However, the manufacture of a high performance dichroic mirror is difficult and costly. It needs a large amount of labour and a long time. Practically the light separation performance of the dichroic mirror is not perfect, and usually several percents of reflected light or transmitted light still remain. Because of it, when the photo-taking light is directed to the film, a portion of the light also enters TV image pick-up tube at the same time, which causes undesirable phenomenons such as image flow on the TV monitor and sticking on the image pick-up tube.
Apart from the above-mentioned problems, there is another problem related to the visual-line-fixing lamp of the fundus camera.
As is well known in the art, the visual-line-fixing lamp is used to guide the line of vision in a determined direction. In the case of a "mydriatic type" eye fundus camera, an external visual-line-fixing lamp is conventionally used for this purpose. In examining one of the eyes of a patient, the other eye is looking at the lamp. In this case, since the pupil of the eye to be examined has previously been diluted with a mydriatic agent, there is never produced such a problem that the examination of the eye is disturbed by miosis caused by any "near reflex" of the eye. In contrast, in the case of the non-mydriatic eye fundus camera where a picture-taking of the eye fundus is carried out without the aid of a mydriatic agent, there is a possibility that the examination is disturbed by miosis caused by near reflex. For this reason, it is preferred to use an internal visual-line-fixing lamp for non-mydriatic type fundus camera (see U.S. Pat. No. 4,068,932). For an eye fundus camera which can be used as both the mydriatic type and non-mydriatic type, however, the use of an internal visual-line-fixing lamp brings forth a problem. That is, if a separate light guiding system is fixedly mounted in the camera in order to eliminate such a troublesome work of demounting the optical system for guiding light to the image pick-up tube by which an invisible image is transformed into a visible image and remounting it onto a direct view-finder, it is required to intercept any external light entering through the direct view-finder when a picture of the eye fundus is to be taken using the internal visual-line-fixing lamp and without mydriatic agent (when the camera is used in its non-mydriatic mode). Therefore the fundus camera is required to have a shutter or a light path change-over mechanism for changing over the light path from finder to lamp or vice versa. Obviously the provision of such an additional element renders the camera complicated in structure and large in size.